In the field of electronics, and in particular in the field of wireless communication, there are many applications wherein bandpass signals may be generated and amplified. A bandpass signal is an electric signal wherein the spectral energy is limited to a specific bandwidth around a carrier frequency. A bandpass signal has no DC component and no spectral components above a certain cutoff frequency. The bandwidth typically amounts to a specific percentage of the carrier frequency.
In most applications, a bandpass signal is generated by means of digital signal processing, the signals being represented as complex-valued digital baseband signals. A digital baseband signal has two components. A real part and an imaginary part, or an I component and a Q component. Typically, the digital I and Q signals are converted to real-valued analog low-pass signals and transferred to the bandpass domain by means of an IQ mixer or a vector modulator operated with a harmonic signal of a carrier frequency. Accordingly, the IQ mixer may be regarded as a frequency converter, and its function may be regarded as frequency conversion.
Generally, a bandpass signal has a non-constant envelope determined by the peak-to-mean ratio. In many cases, the bandpass signal may be amplified by means of an amplifier device.
A power amplifier is a device comprising two terminals, one terminal for the input signal and one terminal for the output signal. An auxiliary power source is used for generating an output signal that has a higher power as compared to the input signal.
An amplifier is implemented by means of amplifier devices such as transistors or tubes. Said amplifier devices are typically nonlinear. In most technical applications such as wireless communication, for example, it is useful to avoid nonlinear distortion of the output signal, however, since said distortions create undesired emissions outside of as well as distortions within the frequency band.
An almost perfectly linear behavior is achieved by operating the amplifier element with small signal amplitudes as compared to the amplitude which is maximally admissible for said element. An increase in the input signal amplitude results in a further increase of the nonlinear behavior, the highest signal values within the output signal being slightly compressed. However, the output signal still has a variable value, and the amplifier may be referred to as being weakly nonlinear.
In contrast to this, a highly nonlinear behavior results in a firmly limited amplitude of the output signal, said output signal being constant and independent of the input quantity. Such a signal merely has two states which correspond with the sign function of the input signal, and it will be referred to as a binary signal below.
Examples of amplifiers comprising almost no or only slight nonlinearity are class-A or class-B amplifiers. Examples of highly nonlinear amplifiers are class-D and class-E amplifiers such as digital line drivers and pulse amplifiers. Said strictly nonlinear amplifiers will be referred to as switching amplifiers below since the amplifier essentially acts as a switching current source triggered by the sign function of the input signal.
The efficiency of an amplifier is defined as the ratio of the average power of the output signal to the input power, the input power in turn being the average power provided by the auxiliary power source and added to the power of the input signal. The efficiency of a weakly nonlinear amplifier is comparatively low and decreases further for signals having a high peak-to-mean ratio.
In contrast to this, switching amplifiers exhibit an efficiency, or efficiency factor, of almost 1, which is due to the fact that ideally, the voltage applied across, or the current flowing through, the switching element is zero at any time. From the point of view of energy efficiency, a switching amplifier is therefore advantageous.
However, what is disadvantageous is that a switching amplifier by definition removes any amplitude information from the input signal. This is why the amplitude of the input signal may be maintained by additionally using a pulse modulation scheme at the input of the amplifier, which results in a binary input signal, any information being inserted in the zero crossings of the signal.
On the output side, additional demodulation of the amplified binary signal may be used for reconstructing the original signal. In addition, modulation creates undesired spectral components outside the frequency band, which may expediently be suppressed by demodulation. On the other hand, the entire system—modulator, switching amplifier, demodulator—would by definition not behave like a linear power amplifier.
In order to obtain undistorted, i.e., linear, amplification of the signal curve, the amplifier device may expediently be operated with a certain headroom, i.e., with a power back-off, which in turn reduces the power efficiency of the amplifier, however. Otherwise, the amplifier will distort the bandpass signal in a nonlinear manner and thus produce undesired distortions within the frequency band as well as emissions outside the frequency band.
The insufficient power efficiency of amplifiers for bandpass signals, which in modern wireless communication signals may amount to only 10 percent or less, results in an increased need for new amplifier concepts. Current approaches employing adaptive digital filter pre-distortions demonstrate that there is a considerably augmented need for using hardware. This is why, additionally, there is a need for integrated solutions involving reduced hardware complexity.
Several approaches have been known in terms of increasing the power efficiency for bandpass amplifiers. At the circuit level, Doherty topology provides higher efficiency for high-power back-off values. Elimination and restoration of envelopes, as well as bias voltage modulation are further techniques at the system level which are aimed at increasing efficiency within the back-off region. Analog feed-forward and feed-back circuitry may be used for improving the linearity of low back-offs, which will also increase power efficiency. In contrast thereto, closed-loop digital pre-distortion is implemented at the system level and exhibits more pronounced improvements in the linearity for signals having comparatively large bandwidths. In addition, it is suited to adapt temporal changes in the amplifier device.
While the approaches mentioned are designed to improve amplifiers having essentially linear behavior, the switching-amplifier techniques are intended to transfer bandpass signals to a sequence of rectangular, binary pulses which may be amplified with a theoretical efficiency of 100 percent. Following amplification, the original bandpass signal is reconstructed by means of low-pass filtering.
From a technical point of view, transformation and reconstruction may be considered to be a pulse modulation and demodulation problem. One known switching-amplifier technique uses pulse width modulation (PWM), which is a feed-forward modulation scheme. A further known switching-amplifier technique uses Sigma-Delta modulation (SDM), which is a closed-loop modulation scheme.
However, both have the disadvantage of causing signal distortions within the frequency band which cannot be removed by the demodulator. Said signal distortions may be reduced by increasing the switching frequency, by oversampling or by means of higher-order loop-back filtering (for SDM). This increases the requirement placed upon the amplifier device and, therefore, the cost of switching amplifiers, specifically for operation at high signal frequencies, which are typical at radio frequencies and in microwave applications. In addition, due to its feedback delay the closed-loop architecture of Sigma-Delta modulation tends to create problems of instability at very high operating frequencies.